Method and apparatus for initial cell search and selection

ABSTRACT

A method performed by a base station may include transmitting a configuration message including at least information indicating a subset of a plurality of transmit beams to be used for transmitting a set of synchronization signals. The set of synchronization signals including a primary synchronization signal and a secondary synchronization signal may be transmitted. A random access channel (RACH) transmission may be received using a receive beam associated with one of the subset of the plurality of transmit beams used by the base station to transmit the set of synchronization signals. The transmitted set of synchronization signals transmitted may have a signal quality above a signal quality threshold. A reference signal may be transmitted along with a physical broadcast channel (PBCH) transmission. The reference signal may have a sequence derived from a beam index associated with the one of the subset of the plurality of transmit beams.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/735,327 filed on Dec. 11, 2017, which issued as U.S. Pat. No.11,336,500 on May 17, 2022, which is the U.S. National Stage, under 35U.S.C. § 371, of International Application No. PCT/US2016/039321 filedJun. 24, 2016, which claims the benefit of U.S. Provisional ApplicationNo. 62/184,580, filed Jun. 25, 2015, and U.S. Provisional ApplicationNo. 62/307,005, which was filed on Mar. 11, 2016, the contents of whichare hereby incorporated by reference herein.

BACKGROUND

The next generation of cellular communication systems, commonly referredto as 5G, will require higher throughput (e.g., up to 10 Gbps or betterover the air) and lower latency (e.g., 1 ms) than previous generations.In order to meet these requirements, additional bandwidth may be needed.One band that may be available for use by 5G cellular communicationsystems that is currently not being used for such purposes is themillimeter wave (mmW) band, which includes frequencies at or above 6GHz. Use of such bands for cellular systems may allow for much higherdata rates than are currently possible and may allow for use of asmaller transmit time interval (TTI), which may reduce latency.

SUMMARY

Methods and apparatus for initial cell search and selection usingbeamforming are described. An apparatus is configured with multiplereceive beams and includes an antenna and a processor. The processor isoperatively coupled to the antenna and sweeps a respective one of themultiple receive beams during each of multiple synchronizationsub-frames, using a pre-defined sweep time and dwell period, to detect asynchronization signal. The processor also obtains symbol timinginformation and a synchronization signal index from the detectedsynchronization signal. The obtained synchronization signal indexcorresponds to a synchronization signal index of the set. The processordecodes a first broadcast channel using the obtained symbol timinginformation, the obtained synchronization signal index and a predefinedor blind-coded symbol distance between the detected synchronizationsignal and the first broadcast channel. The processor decodes a secondbroadcast channel using information obtained from decoding the firstbroadcast channel.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1A is a system diagram of an example communications system in whichone or more disclosed embodiments may be implemented;

FIG. 1B is a system diagram of an example wireless transmit/receive unit(WTRU) that may be used within the communications system illustrated inFIG. 1A;

FIG. 1C is a system diagram of an example radio access network and anexample core network that may be used within the communications systemillustrated in FIG. 1A;

FIG. 2 is a diagram of an example frequency division duplex (FDD) frameshowing example primary synchronization signal (PSS) and secondarysynchronization signal (SSS) locations;

FIG. 3 is a diagram of another example mmW frame structure;

FIG. 4 is a diagram of an example of a 3-sector mmW base station siteusing M beams per cell and an example WTRU with N total receive breams;

FIG. 5 is a diagram showing signal beam coverage examples with singlebeam configuration;

FIG. 6 is a diagram showing signal beam coverage examples with twosimultaneous beams configured;

FIG. 7 is a diagram of example sub-frames that use uniform and fullsweep in one synchronization region;

FIG. 8 is a diagram of example sub-frames that use uniform and fullsweep in a multiple-TTI synchronization region;

FIG. 9 is a diagram of an example synchronization regionre-configuration;

FIG. 10 is a diagram of another example synchronization regionre-configuration;

FIG. 11 is a diagram of an example of a flexible mapping and dwell timereconfiguration;

FIG. 12 is a diagram of an example mapping of synchronization signals toheterogeneous eNB transmit beams;

FIG. 13 is a diagram of an example mapping of synchronization signals toheterogeneous transmit beams in different TTIs;

FIG. 14 is a diagram of an example mapping of synchronization signaltypes to heterogeneous eNB transmit beams at the same symbols;

FIG. 15 is a diagram of an example mapping where the master informationblock (MIB) is divided such that the beam information is transmitted ona cell-wide eNB transmit beam and the rest of the MIB is transmitted ina narrow transmit beam;

FIG. 16 is a diagram of an example mapping where the broadcast channelis associated with a synchronization signal type in a one-to-onemapping;

FIG. 17 is a diagram of an example mapping where the broadcast channelis associated with a synchronization signal type in a one-to-manymapping;

FIG. 18 is a flow diagram of an example method of initial cell searchand selection using beamforming;

FIG. 19 is a diagram showing examples of an exhaustive search and astaged search;

FIG. 20 is a graph showing results of a first set of simulationsstudying impact of rotation using exhaustive and staged searchprocedures; and

FIG. 21 is a graph showing results of a second set of simulationsstudying the performance of the exhaustive and staged search proceduresas a function of rotation speed.

DETAILED DESCRIPTION

FIG. 1A is a diagram of an example communications system 100 in whichone or more disclosed embodiments may be implemented. The communicationssystem 100 may be a multiple access system that provides content, suchas voice, data, video, messaging, broadcast, etc., to multiple wirelessusers. The communications system 100 may enable multiple wireless usersto access such content through the sharing of system resources,including wireless bandwidth. For example, the communications systems100 may employ one or more channel access methods, such as code divisionmultiple access (CDMA), time division multiple access (TDMA), frequencydivision multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrierFDMA (SC-FDMA), and the like.

As shown in FIG. 1A, the communications system 100 may include wirelesstransmit/receive units (WTRUs) 102 a, 102 b, 102 c, 102 d, a radioaccess network (RAN) 104, a core network 106, a public switchedtelephone network (PSTN) 108, the Internet 110, and other networks 112,though it will be appreciated that the disclosed embodiments contemplateany number of WTRUs, base stations, networks, and/or network elements.Each of the WTRUs 102 a, 102 b, 102 c, 102 d may be any type of deviceconfigured to operate and/or communicate in a wireless environment. Byway of example, the WTRUs 102 a, 102 b, 102 c, 102 d may be configuredto transmit and/or receive wireless signals and may include userequipment (UE), a mobile station, a fixed or mobile subscriber unit, apager, a cellular telephone, a personal digital assistant (PDA), asmartphone, a laptop, a netbook, a personal computer, a wireless sensor,consumer electronics, and the like.

The communications systems 100 may also include a base station 114 a anda base station 114 b. Each of the base stations 114 a, 114 b may be anytype of device configured to wirelessly interface with at least one ofthe WTRUs 102 a, 102 b, 102 c, 102 d to facilitate access to one or morecommunication networks, such as the core network 106, the Internet 110,and/or the other networks 112. By way of example, the base stations 114a, 114 b may be a base transceiver station (BTS), a Node-B, an eNode B,a Home Node B, a Home eNode B, a site controller, an access point (AP),a wireless router, and the like. While the base stations 114 a, 114 bare each depicted as a single element, it will be appreciated that thebase stations 114 a, 114 b may include any number of interconnected basestations and/or network elements.

The base station 114 a may be part of the RAN 104, which may alsoinclude other base stations and/or network elements (not shown), such asa base station controller (BSC), a radio network controller (RNC), relaynodes, etc. The base station 114 a and/or the base station 114 b may beconfigured to transmit and/or receive wireless signals within aparticular geographic region, which may be referred to as a cell (notshown). The cell may further be divided into cell sectors. For example,the cell associated with the base station 114 a may be divided intothree sectors. Thus, in one embodiment, the base station 114 a mayinclude three transceivers, i.e., one for each sector of the cell. Inanother embodiment, the base station 114 a may employ multiple-inputmultiple-output (MIMO) technology and, therefore, may utilize multipletransceivers for each sector of the cell.

The base stations 114 a, 114 b may communicate with one or more of theWTRUs 102 a, 102 b, 102 c, 102 d over an air interface 116, which may beany suitable wireless communication link (e.g., radio frequency (RF),microwave, infrared (IR), ultraviolet (UV), visible light, etc.). Theair interface 116 may be established using any suitable radio accesstechnology (RAT).

More specifically, as noted above, the communications system 100 may bea multiple access system and may employ one or more channel accessschemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. Forexample, the base station 114 a in the RAN 104 and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access (UTRA), whichmay establish the air interface 116 using wideband CDMA (WCDMA). WCDMAmay include communication protocols such as High-Speed Packet Access(HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-Speed DownlinkPacket Access (HSDPA) and/or High-Speed Uplink Packet Access (HSUPA).

In another embodiment, the base station 114 a and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Evolved UMTSTerrestrial Radio Access (E-UTRA), which may establish the air interface116 using Long Term Evolution (LTE) and/or LTE-Advanced (LTE-A).

In other embodiments, the base station 114 a and the WTRUs 102 a, 102 b,102 c may implement radio technologies such as IEEE 802.16 (i.e.,Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000,CDMA2000 1×, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), InterimStandard 95 (IS-95), Interim Standard 856 (IS-856), Global System forMobile communications (GSM), Enhanced Data rates for GSM Evolution(EDGE), GSM EDGE (GERAN), and the like.

The base station 114 b in FIG. 1A may be a wireless router, Home Node B,Home eNode B, or access point, for example, and may utilize any suitableRAT for facilitating wireless connectivity in a localized area, such asa place of business, a home, a vehicle, a campus, and the like. In oneembodiment, the base station 114 b and the WTRUs 102 c, 102 d mayimplement a radio technology such as IEEE 802.11 to establish a wirelesslocal area network (WLAN). In another embodiment, the base station 114 band the WTRUs 102 c, 102 d may implement a radio technology such as IEEE802.15 to establish a wireless personal area network (WPAN). In yetanother embodiment, the base station 114 b and the WTRUs 102 c, 102 dmay utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE,LTE-A, etc.) to establish a picocell or femtocell. As shown in FIG. 1A,the base station 114 b may have a direct connection to the Internet 110.Thus, the base station 114 b may not be required to access the Internet110 via the core network 106.

The RAN 104 may be in communication with the core network 106, which maybe any type of network configured to provide voice, data, applications,and/or voice over internet protocol (VoIP) services to one or more ofthe WTRUs 102 a, 102 b, 102 c, 102 d. For example, the core network 106may provide call control, billing services, mobile location-basedservices, pre-paid calling, Internet connectivity, video distribution,etc., and/or perform high-level security functions, such as userauthentication. Although not shown in FIG. 1A, it will be appreciatedthat the RAN 104 and/or the core network 106 may be in direct orindirect communication with other RANs that employ the same RAT as theRAN 104 or a different RAT. For example, in addition to being connectedto the RAN 104, which may be utilizing an E-UTRA radio technology, thecore network 106 may also be in communication with another RAN (notshown) employing a GSM radio technology.

The core network 106 may also serve as a gateway for the WTRUs 102 a,102 b, 102 c, 102 d to access the PSTN 108, the Internet 110, and/orother networks 112. The PSTN 108 may include circuit-switched telephonenetworks that provide plain old telephone service (POTS). The Internet110 may include a global system of interconnected computer networks anddevices that use common communication protocols, such as thetransmission control protocol (TCP), user datagram protocol (UDP) andthe internet protocol (IP) in the TCP/IP internet protocol suite. Thenetworks 112 may include wired or wireless communications networks ownedand/or operated by other service providers. For example, the networks112 may include another core network connected to one or more RANs,which may employ the same RAT as the RAN 104 or a different RAT.

Some or all of the WTRUs 102 a, 102 b, 102 c, 102 d in thecommunications system 100 may include multi-mode capabilities, i.e., theWTRUs 102 a, 102 b, 102 c, 102 d may include multiple transceivers forcommunicating with different wireless networks over different wirelesslinks. For example, the WTRU 102 c shown in FIG. 1A may be configured tocommunicate with the base station 114 a, which may employ acellular-based radio technology, and with the base station 114 b, whichmay employ an IEEE 802 radio technology.

FIG. 1B is a system diagram of an example WTRU 102. As shown in FIG. 1B,the WTRU 102 may include a processor 118, a transceiver 120, atransmit/receive element 122, a speaker/microphone 124, a keypad 126, adisplay/touchpad 128, non-removable memory 130, removable memory 132, apower source 134, a global positioning system (GPS) chipset 136, andother peripherals 138. It will be appreciated that the WTRU 102 mayinclude any sub-combination of the foregoing elements while remainingconsistent with an embodiment.

The processor 118 may be a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Array (FPGAs)circuits, any other type of integrated circuit (IC), a state machine,and the like. The processor 118 may perform signal coding, dataprocessing, power control, input/output processing, and/or any otherfunctionality that enables the WTRU 102 to operate in a wirelessenvironment. The processor 118 may be coupled to the transceiver 120,which may be coupled to the transmit/receive element 122. While FIG. 1Bdepicts the processor 118 and the transceiver 120 as separatecomponents, it will be appreciated that the processor 118 and thetransceiver 120 may be integrated together in an electronic package orchip.

The transmit/receive element 122 may be configured to transmit signalsto, or receive signals from, a base station (e.g., the base station 114a) over the air interface 116. For example, in one embodiment, thetransmit/receive element 122 may be an antenna configured to transmitand/or receive RF signals. In another embodiment, the transmit/receiveelement 122 may be an emitter/detector configured to transmit and/orreceive IR, UV, or visible light signals, for example. In yet anotherembodiment, the transmit/receive element 122 may be configured totransmit and receive both RF and light signals. It will be appreciatedthat the transmit/receive element 122 may be configured to transmitand/or receive any combination of wireless signals.

In addition, although the transmit/receive element 122 is depicted inFIG. 1B as a single element, the WTRU 102 may include any number oftransmit/receive elements 122. More specifically, the WTRU 102 mayemploy MIMO technology. Thus, in one embodiment, the WTRU 102 mayinclude two or more transmit/receive elements 122 (e.g., multipleantennas) for transmitting and receiving wireless signals over the airinterface 116.

The transceiver 120 may be configured to modulate the signals that areto be transmitted by the transmit/receive element 122 and to demodulatethe signals that are received by the transmit/receive element 122. Asnoted above, the WTRU 102 may have multi-mode capabilities. Thus, thetransceiver 120 may include multiple transceivers for enabling the WTRU102 to communicate via multiple RATs, such as UTRA and IEEE 802.11, forexample.

The processor 118 of the WTRU 102 may be coupled to, and may receiveuser input data from, the speaker/microphone 124, the keypad 126, and/orthe display/touchpad 128 (e.g., a liquid crystal display (LCD) displayunit or organic light-emitting diode (OLED) display unit). The processor118 may also output user data to the speaker/microphone 124, the keypad126, and/or the display/touchpad 128. In addition, the processor 118 mayaccess information from, and store data in, any type of suitable memory,such as the non-removable memory 130 and/or the removable memory 132.The non-removable memory 130 may include random-access memory (RAM),read-only memory (ROM), a hard disk, or any other type of memory storagedevice. The removable memory 132 may include a subscriber identitymodule (SIM) card, a memory stick, a secure digital (SD) memory card,and the like. In other embodiments, the processor 118 may accessinformation from, and store data in, memory that is not physicallylocated on the WTRU 102, such as on a server or a home computer (notshown).

The processor 118 may receive power from the power source 134, and maybe configured to distribute and/or control the power to the othercomponents in the WTRU 102. The power source 134 may be any suitabledevice for powering the WTRU 102. For example, the power source 134 mayinclude one or more dry cell batteries (e.g., nickel-cadmium (NiCd),nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion),etc.), solar cells, fuel cells, and the like.

The processor 118 may also be coupled to the GPS chipset 136, which maybe configured to provide location information (e.g., longitude andlatitude) regarding the current location of the WTRU 102. In additionto, or in lieu of, the information from the GPS chipset 136, the WTRU102 may receive location information over the air interface 116 from abase station (e.g., base stations 114 a, 114 b) and/or determine itslocation based on the timing of the signals being received from two ormore nearby base stations. It will be appreciated that the WTRU 102 mayacquire location information by way of any suitablelocation-determination method while remaining consistent with anembodiment.

The processor 118 may further be coupled to other peripherals 138, whichmay include one or more software and/or hardware modules that provideadditional features, functionality and/or wired or wirelessconnectivity. For example, the peripherals 138 may include anaccelerometer, an e-compass, a satellite transceiver, a digital camera(for photographs or video), a universal serial bus (USB) port, avibration device, a television transceiver, a hands free headset, aBluetooth® module, a frequency modulated (FM) radio unit, a digitalmusic player, a media player, a video game player module, an Internetbrowser, and the like.

FIG. 1C is a system diagram of the RAN 104 and the core network 106according to an embodiment. As noted above, the RAN 104 may employ anE-UTRA radio technology to communicate with the WTRUs 102 a, 102 b, 102c over the air interface 116. The RAN 104 may also be in communicationwith the core network 106.

The RAN 104 may include eNode-Bs 140 a, 140 b, 140 c, though it will beappreciated that the RAN 104 may include any number of eNode-Bs whileremaining consistent with an embodiment. The eNode-Bs 140 a, 140 b, 140c may each include one or more transceivers for communicating with theWTRUs 102 a, 102 b, 102 c over the air interface 116. In one embodiment,the eNode-Bs 140 a, 140 b, 140 c may implement MIMO technology. Thus,the eNode-B 140 a, for example, may use multiple antennas to transmitwireless signals to, and receive wireless signals from, the WTRU 102 a.

Each of the eNode-Bs 140 a, 140 b, 140 c may be associated with aparticular cell (not shown) and may be configured to handle radioresource management decisions, handover decisions, scheduling of usersin the uplink and/or downlink, and the like. As shown in FIG. 1C, theeNode-Bs 140 a, 140 b, 140 c may communicate with one another over an X2interface.

The core network 106 shown in FIG. 1C may include a mobility managemententity gateway (MME) 142, a serving gateway 144, and a packet datanetwork (PDN) gateway 146. While each of the foregoing elements aredepicted as part of the core network 106, it will be appreciated thatany one of these elements may be owned and/or operated by an entityother than the core network operator.

The MME 142 may be connected to each of the eNode-Bs 140 a, 140 b, 140 cin the RAN 104 via an S1 interface and may serve as a control node. Forexample, the MME 142 may be responsible for authenticating users of theWTRUs 102 a, 102 b, 102 c, bearer activation/deactivation, selecting aparticular serving gateway during an initial attach of the WTRUs 102 a,102 b, 102 c, and the like. The MME 142 may also provide a control planefunction for switching between the RAN 104 and other RANs (not shown)that employ other radio technologies, such as GSM or WCDMA.

The serving gateway 144 may be connected to each of the eNode Bs 140 a,140 b, 140 c in the RAN 104 via the S1 interface. The serving gateway144 may generally route and forward user data packets to/from the WTRUs102 a, 102 b, 102 c. The serving gateway 144 may also perform otherfunctions, such as anchoring user planes during inter-eNode B handovers,triggering paging when downlink data is available for the WTRUs 102 a,102 b, 102 c, managing and storing contexts of the WTRUs 102 a, 102 b,102 c, and the like.

The serving gateway 144 may also be connected to the PDN gateway 146,which may provide the WTRUs 102 a, 102 b, 102 c with access topacket-switched networks, such as the Internet 110, to facilitatecommunications between the WTRUs 102 a, 102 b, 102 c and IP-enableddevices.

The core network 106 may facilitate communications with other networks.For example, the core network 106 may provide the WTRUs 102 a, 102 b,102 c with access to circuit-switched networks, such as the PSTN 108, tofacilitate communications between the WTRUs 102 a, 102 b, 102 c andtraditional land-line communications devices. For example, the corenetwork 106 may include, or may communicate with, an IP gateway (e.g.,an IP multimedia subsystem (IMS) server) that serves as an interfacebetween the core network 106 and the PSTN 108. In addition, the corenetwork 106 may provide the WTRUs 102 a, 102 b, 102 c with access to thenetworks 112, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

Other network 112 may further be connected to an IEEE 802.11 basedwireless local area network (WLAN) 160. The WLAN 160 may include anaccess router 165. The access router may contain gateway functionality.The access router 165 may be in communication with a plurality of accesspoints (APs) 170 a, 170 b. The communication between access router 165and APs 170 a, 170 b may be via wired Ethernet (IEEE 802.3 standards),or any type of wireless communication protocol. AP 170 a is in wirelesscommunication over an air interface with WTRU 102 d.

Above-6 GHz frequencies have traditionally not been used for cellularsystems due to propagation characteristics that have been presumed to beunfavorable for wireless communication in outdoor environments. Higherfrequency transmissions generally tend to experience higher free spacepath loss. Rainfall, atmospheric gasses (e.g. oxygen), and foliage mayadd further attenuation compared to sub-6 GHz frequencies. In addition,penetration and diffraction attenuation may become more severe at mmWfrequencies as opposed to sub-6 GHz frequencies.

The above-described propagation characteristics of above-6 GHzfrequencies may result in significant Non Line-Of-Sight (NLOS)propagation path loss. For example, at mmW frequencies, NLOS path lossmay be more than 20 dB higher than Line-Of-Sight (LOS) path loss and mayseverely limit the coverage of the mmW transmission.

Recent channel measurements have demonstrated feasibility of outdoor mmWcellular coverage with the help of beamforming techniques. Themeasurement data shows that the beamforming gain may not only be able toprovide required coverage for cellular control signaling in NLOSconditions but may also boost the link capacity to achieve higher datathroughput in LOS conditions. Antennas that implement such beamformingtechniques may need to provide high gain and, therefore, be highlydirectional, which may require use of large antenna arrays that areelectronically steerable at both the transmitter and receiver.

Given the propagation characteristics of above-6 GHz frequency channelsand the very high data throughput requirements of 5G cellular systems,5G systems may be optimally designed to enable a beamformed access linkwith beamforming on all physical layer signals and channels. Thephysical layer signals and channels may apply different beamformingtechniques and may also have their own specific beamformingconfiguration (e.g., beamwidth and beamforming gain). Further, above6-GHz system designs may incorporate beamforming aspects into all systemprocedures. An aligned beam pair at above 6-GHz frequencies may providean additional degree of freedom in the angular domain compared withconventional cellular systems. The system design may take into accountthe beamforming and beam pairing features specific to each physicallayer signal and channel and incorporate the corresponding spatialcontrol and maneuvering into all system procedures, including, forexample, cell search, random access, and control channel decoding.

Beamforming techniques may include digital, analog or hybridbeamforming. With digital beamforming, each antenna element may have adedicated radio frequency (RF) chain, each of which may include RFprocessing elements and analog-to-digital/digital-to-analog converters(ADC/DAC). The signal processed by each antenna element may becontrolled independently in phase and amplitude to optimize the channelcapacity. The number of RF chains may be equal to the number of antennaelements. While offering very high performance, digital beamformingtechniques may impose a high cost and complexity in implementation andcause high energy consumption in operation.

Analog beamforming may require only one RF chain for a number of antennaelements that constitute a Phase Antenna Array (PAA). Each antennaelement may have a phase shifter, which may be used to set a phase-onlyweight for beamforming and steering of the antenna pattern of the PAA.The number of applied RF chains may be significantly lower than thenumber of antenna elements, and the number of RF chains may be the sameas, or lower than, the number of PAAs. For example, multiple PAAs may beconnected to a single RF chain, and each PAA may have an antenna patternof specific azimuth and elevation coverage. The RF chain may be switchedto one PAA at a time and thus a single RF chain with multiple PAAs mayprovide a broad coverage by using one beam at a different direction at adifferent time instance.

Hybrid beamforming may combine digital precoding and analog beamforming.The analog beamforming may be performed over antenna elements of a PAAconnected to one RF chain. The digital precoding may be applied to thebaseband signal for each RF chain and its associated PAA. Theconfiguration of the hybrid beamforming may include a number of datastreams, a number of RF chains, a number of a PAAs and a number ofantenna elements. One PAA connected to an RF chain may be represented byan antenna port uniquely identified by a beamformed reference signalspecific to the antenna port.

The high implementation cost and energy consumption of digitalbeamforming techniques for above-6 GHz systems may introduce specificimplementation considerations for an above-6 GHz 5G wireless system. Forexample, the above-6 GHz 5G beamforming technique may be based on hybridbeamforming with a high degree of analog beamforming such that, forexample, the number of RF chains may be significantly lower than thenumber of antenna elements. Implications of the analog beamformingtechnique may impact all system procedures, including initial cellsearch, and may result in new procedural behaviors and events.Directional transmission may offer a high degree of flexibility to theeNB to customize the transmission both in the time and spatial domainsto reduce signal overhead and energy consumption.

Initial cell search is a procedure through which a WTRU may attempt togain initial access to a network by acquiring time and frequencysynchronization with a cell and detecting the Cell ID of the cell. Theprocedure may be facilitated by one or more synchronization signals,which may be transmitted by all cells in the network. Thesynchronization signals may include, for example a primarysynchronization signal (PSS) and a secondary synchronization signal(SSS).

FIG. 2 is a diagram of an example Long Term Evolution (LTE) frequencydivision duplex (FDD) frame 200 showing example PSS and SSS locations.As illustrated in FIG. 2 , the PSS 205 and SSS 210 are transmitted inthe 0th subframe 220 and the 5th subframe 230 of every radio frame 200and may be used for time and frequency synchronization duringinitialization. The frame 200 may be a 10 ms frame and may be dividedinto 10 subframes of 1 ms each.

The synchronization signals may be based on Zadoff-Chu (ZC) sequencesand may be used by the WTRU to find an appropriate cell within thenetwork, determine its downlink frame timing, and identify its physicallayer identity. More specifically, as part of the system acquisitionprocess, a WTRU may synchronize sequentially to the OFDM symbol, slot,subframe, half-frame, and radio frame based on the synchronizationsignals. In LTE, for example, the PSS may be used to obtain slot,subframe and half-frame boundary synchronization. The PSS may alsoprovide physical layer (PHY) cell identity (PCI) within the cellidentity group. In LTE, for example, there are 504 different PCIs, whichare divided into 168 groups, each associated with three differentidentities that are mapped to three possible PSS sequences. Thisgrouping may reduce the complexity of the overall cell search procedure.The SSS may be used to obtain the radio frame boundary and may alsoenable the WTRU to determine the cell identity group, which may rangefrom 0 to 167. Following a successful synchronization and PCIacquisition, a WTRU may, for example, decode the physical broadcastchannel (PBCH) with the help of a cell-specific reference signal (CRS)and acquire master information block (MIB) information regarding systembandwidth, system frame number (SFN) and physical hybrid automaticrepeat request (HARQ) indicator channel (PHICH) configuration.

The initial search procedure described above assumes a fixed beampattern with cell-wide coverage. For example, the LTE synchronizationsignals and PBCH described above are transmitted continuously accordingto standardized periodicity. However, for mmW networks employingdual-end highly directional links, using analog or hybrid beamformingtechniques to minimize the number of RF chains utilized, such coveragemay not be practical. Embodiments described herein provide methods andapparatus for initial cell search and selection where beamforming mayemployed at both the network and WTRU.

The initial search procedure described above also assumes use of anomnidirectional antenna pattern, which allows for WTRU rotational motionto be largely ignored. However, for mmW systems, support for highlydirectional links may require the antennas to be electronicallysteerable, and mmW systems may also require a beam pairing to beestablished. Both of these aspects of mmW systems may make them moresensitive to WTRU rotational motion. For example, a beam pair tested atthe beginning of an exhaustive search procedure may not have the samequality at the end of the search when the beam pairing decision is made.Embodiments described herein provide methods of initial cell search andselection where beamforming may be applied at both the network and theWTRU, which take into account rotational motion of the WTRU.

At the network end, methods and apparatus are described that may enablea network to provide the synchronization signals and PBCH in a manner inwhich mmW WTRUs (also referred to herein simply as WTRUs) may receivethem. In embodiments, in order to reach users at cell edge and serviceall users in high density cells, the synchronization and broadcastchannels may be transmitted on multiple beams, which may not betransmitted continuously in time. In embodiments, the PSS and SSS may betransmitted on different beams. Similarly, the PSS/SSS and PBCH may betransmitted on different beams.

Embodiments described herein provide for mmW synchronization signaldesign and methods of mapping one or more synchronization signal types(e.g., PSS and SSS) to physical resources. The one or moresynchronization signal types may provide one or more pieces of timinginformation, including, for example, symbol timing, TTI timing, subframetiming and frame timing.

In embodiments, a mmW system may use a sub-frame of the same length asthe LTE system (e.g., as illustrated in FIG. 2 ), and sub-frame timingalignment may, for example, facilitate dual connectivity. In suchembodiments, each subframe may have a number of TTIs due to the widerbandwidth afforded by use of the mmW band. In embodiments, a shorter TTIlength (e.g., 100 μs) may be used to reduce latency.

FIG. 3 is a diagram of another example mmW frame structure. The exampleframe 300 illustrated in FIG. 3 is 1 ms in length and includes 10subframes of 0.1 ms each. The example frame 300 accommodates a number oftransmit beams, M, per synchronization sub-frame 320, 330 and has asub-frame periodicity of one synchronization sub-frame 320, 330 perframe 300. In the illustrated example, the synchronization signals arelocated in the first sub-frame 320, 330 of each downlink frame 300 andare mapped to the M beams 305-310, as described in more detail below. Invarious embodiments, different numbers of beams M may be used persub-frame and different sub-frame periodicities may be used.

FIG. 4 is a diagram of an example system 400 including a 3-sector mmWbase station site 410 using M beams 305-310 per each of the three cells450A, 450B and 450C and a WTRU 420 with N total receive breams 440.Although traditional hexagonal cell coverage is shown, mmW coverage maybe less structured, for example, due to the effects of blockages. FIG. 4also highlights an example beam pairing between receive beam 440A (beamindex 1) of the WTRU 420 and eNB transmit beam 430 (beam index 2) fromthe cell 450C. For simplicity, FIG. 4 shows the WTRU 420 having a singlearray. However, to increase coverage, a WTRU may have more than onearray.

In embodiments, a WTRU may use specific sequences mapped to a PSS and/orSSS to determine the beam and/or cell identity. In the embodimentillustrated in FIG. 3 , for example, each of the M beams may carry botha common cell identity and a unique beam specific identity. Inembodiments, each sequence may carry a signature that uniquelyidentifies a cell, a beam or a user. For example, a ZC sequence may beused with one ZC root sequence denoting one cell, and each beam of thecell may use a ZC sequence based on the root sequence and apre-configured number of cyclic shifts. In another example, othersequences with desired auto-correlation and cross-correlationproperties, such as a Golay sequence and a Gold sequence, may beconsidered. The sequence index may carry additional system information.For example, the PSS may carry the cell-specific identity, and the SSmay carry the beam-specific identity.

A synchronization signal may also carry specific information that may beused to locate and identify another synchronization signal. For example,the PSS may carry implicit information in terms of a selected index,which may indicate the symbol distance between the PSS and itsassociated SSS. In other embodiments, the symbol distance between thePSS and the associated SSS may be pre-defined. In addition, one or moreproperties, such as resource mapping or scrambling sequenceconfiguration of one synchronization signal type, may be derived basedon its associated synchronization signal.

A synchronization signal may also have a specific beamformingconfiguration and properties, which may include, for example, beamwidthand associated gain, side lobe suppression level and transmit power ofeach antenna element. The beamwidth and properties of a synchronizationsignal may be tailored to the specific synchronization signal thesynchronization signal beam (or beam) is associated with and may befixed or variable. For example, a synchronization signal beam may havedifferent bandwidth and properties depending on whether it is being usedto provide cell-wide coverage or cover a smaller hotspot, for example.

Each synchronization signal may use a set of pre-defined sequences, andthe configuration of the sequences may be different between differentsynchronization signals (e.g., the configuration of the sequences may bedifferent for a PSS than for an SSS). The configuration may include, forexample, sequence type, sequence length, and/or sequence modulation.Attributes of the sequence may include, for example, cell-specificidentity and beam-specific identity. In embodiments, all of thebeamformed synchronization signal types may carry the same cell specificsequence. In other embodiments, all synchronization signal types for onecell may use different sequences from one pre-defined set of sequences,and the sequence set may use certain properties to uniquely identityboth the cell-specific and beam-specific information. For example, allsequences in the set may be based on the same base sequence with adifferent cyclic shift.

The beamforming configuration and periodicity used for a particularsynchronization signal or synchronization signal type may depend on thetype of service it is being used for in order to save energy. Forexample, enhanced broadband related services, such as very faststreaming, may use a different configuration than a low capacity, buthighly reliable, service. Further, depending on user density and traffictype, a cell may use a different beamforming configuration for ahotspot.

Physical resources used for synchronization signals may include a groupof physical resource elements, which may be the minimum schedulablephysical layer resource of the mmW system. Each physical resourceelement may carry one symbol of a PHY control channel, PHY data channel,synchronization signal or reference signal. Further, each physicalresource element may include one or more minimum physical resourceunits, which may be defined and specified in accordance with waveform,modulation and frame structure employed by the mmW system. The minimumresource units may include, for example, a minimum frequency resourceunit, a minimum time resource unit and/or a minimum modulation unit. Theminimum frequency resource unit may be, for example, a subcarrierapplied in a multi-carrier waveform such as OFDM, SC-OFDM, filter bankmulticarrier (FBMC), zero-tail spread-OFDM (ZT-s-OFDM), or unique wordOFDM (UW OFDM). Another example of the minimum frequency resource unitmay be a broad band carrier applied in a time domain single carrierwaveform (SC). The minimum time resource unit may be, for example, atime domain Fast Fourier Transform (FFT) symbol applied in an SCwaveform or an OFDM symbol in OFDM-based waveforms. With regard to theminimum modulation unit, for example, a modulated symbol may usedifferent modulation schemes, such as binary phase shift keying (BPSK),quadrature PSK (QPSK), 16 quadrature amplified modulation (16-QAM) and64-QAM. The modulated symbol may have a sequence of minimum modulationunits that may represent a data symbol. The length of the sequence maydepend on the modulation scheme.

In embodiments, a flexible mapping may be used for mappingsynchronization signals to physical resources. For example, differentsynchronization signal types (e.g., PSS or SSS) may be transmittedaccording to a beam-specific physical resource mapping. The mapping mayinclude, for example, beam-specific synchronization signal physicalresource allocation, dwell time, sweep time and periodicity.

The dwell time of a synchronization signal may be the time thesynchronization signal is transmitted continuously using the same eNBtransmit beam. Each eNB transmit beam may be uniquely defined by thebeam steering vector and its beam coverage. The eNB may perform a sweepby transmitting a synchronization signal in a set of eNB transmit beamsaccording to a pre-determined order or pattern. Depending on eNBcapability, the sweep may apply one single or multiple simultaneousbeams at one time instance. Periodicity may determine the time betweensynchronization signal type transmissions in the same transmit beam andsince the end of the previous transmission.

A synchronization signal configuration may include a maximum sweep timein terms of a time resource unit (e.g., symbol within a TTI, a number ofTTIs or a number of subframes and a beam dwell time in terms of a timeresource unit (e.g., symbol within a TTI)). The sweep time may be, forexample, the number of TTIs required for an eNB to sweep its transmitbeams to cover the entire cell for one cycle. The maximum number ofbeamformed synchronization signals and their resource mapping may bepredefined. When the number of beams supported by a cell is smaller thanthe maximum number of beams, the eNB may choose to repeat the beams.

FIG. 5 is a diagram 500 showing signal beam coverage examples 502, 504and 506 with a single beam configuration. In the illustrated example502, an eNB 510 transmits a synchronization signal type (e.g., PSS orSSS) in a set of adjacent and consecutive eNB transmit beams 520A, 520B,520C, 520D, 520E in the azimuth plane to provide cell-wide coverage forthat synchronization type. The sweep time may be the sum of the dwelltime of each eNB transmit beam. The eNB may perform another sweep of adifferent pattern with transmit beam coverage in an elevation plane asillustrated in example 504.

In embodiments, the eNB may use a different physical resource mappingfor synchronization signals based, for example, on cell-specificstatistics, such as distribution of user density and traffic types ofdifferent parts of a cell. As illustrated in example 506 in FIG. 5 ,different synchronization signal types may be mapped to transmit beamswith different beam coverage. In addition, the physical resourceallocation, dwell time, beam sweep time and periodicity of thesynchronization signal type may be flexible and adjusted dynamicallyaccording to the statistics. For example, the eNB 510 may configure onesynchronization signal type with a longer dwell time to synchronize withthe cell. On the other hand, the eNB 510 may configure a shorter dwelltime and a larger area when users may be sparse. The dynamicreconfiguration may be signaled, for example, in the system broadcastinformation or using dedicated signaling.

In embodiments, an eNB with multiple RF chains may reconfigure thesynchronization signal type mapping to simultaneous multiple transmitbeams to adapt the synchronization signal transmission based on desiredcell and hotspot coverage, cell-specific statistics including userdensity distribution and traffic type, and other real-time parameters.In embodiments, the eNB may transmit two simultaneous synchronizationsignals, which may be the same or different synchronization signal type.When the simultaneously transmitted synchronization signals are the sametype, for example, they may have different dwell and sweep times. Inembodiments, each synchronization signal may be mapped to a differentand customized beam coverage.

FIG. 6 is a diagram 600 of signal beam coverage examples with twosimultaneous beams 610, 620 configured. As illustrated in examples 650and 690 of FIG. 6 , the eNB 630 may map one synchronization signal typeto two transmit beams 610A, 610E and 620A, 620E with similar beamcoverage area, and the eNB 630 may use these two beams 610A, 610E and620A, 620E to cover different sectors or parts of the cell. Otherconfigurations of the synchronization signal may be beam specific anddifferent, such as with respect to frequency resource allocation, beamdwell time and periodicity. The eNB 630 may configure the mapping toprovide different coverage areas as required, for example, bystatistics, such as user density. In examples 650 and 690, for example,the eNB 630 may apply a longer dwell time in certain areas of the cellon a condition that, for example, the users of the cell are moreconcentrated in these areas.

In an embodiment, such as illustrated in example 660 of FIG. 6 , the eNB630 may transmit one synchronization signal 610B with low power and widebeamwidth for cell-center users and may simultaneously transmit anothersynchronization signal 620B with high power and narrow bandwidth forcell edge users. The eNB 630 may configure different synchronizationsignal type mapping parameters, such as dwell time, sweep time, andsynchronization region, to further customize the synchronizationsignal's coverage and availability.

In the embodiment illustrated in example 670 of FIG. 6 , the eNB 630provides two hotspot coverages 610C, 620C with both of synchronizationsignals with narrow beam and long dwell time, on a condition that, forexample, user distribution in the cell is high in those two areas. Inthe embodiment illustrated in example 680 of FIG. 6 , the eNB 630 mayapply one synchronization signal beam 610D to perform cell-wide coveragewhile using another synchronization signal beam 620D for ad-hoc hotspotcoverage.

As described above, in the frame structure of FIG. 3 , thesynchronization signals are transmitted in a synchronization region inthe first sub-frame 320, 330 of each frame 300, and the frame andsub-frame each have a shortened length of 1 ms and 0.1 ms, respectively.However, in mmW embodiments that use a subframe that has the same lengthas an LTE subframe (e.g., sub-frames are 1 ms in length), as describedabove, the synchronization region may be configured differently. Forexample, the synchronization region may include a number of minimumresource units (e.g., symbols) in one TTI. These symbols may beconsecutive or intermittent, and the synchronization region may startfrom the beginning or end of a TTI. The number of symbols, or the lengthof the synchronization region, may vary depending on, for example, howmany synchronization signal types are to be transmitted and the dwelltime of each synchronization signal beam in each TTI. Thesynchronization region may, thus, be specific to a TTI and may beflexibly set by the eNB depending on the beam availability and/or userdistribution in the cell. The eNB may dynamically configure thesynchronization region, as well as the eNB transmit coverage, beam dwelltime, beam sweep time and periodicity, as described above, to customizeeach synchronization signal type's coverage and availability based onone or more cell-specific statistic, such as desired coverage areas,distribution of user density, distribution of traffic type andinter-beam interference.

The eNB may map one or more synchronization signal types to onesynchronization region or one or more consecutive symbols. The eNB mayconfigure the location of the synchronization region within a TTI, suchas the beginning and end of the TTI, and the duration of thesynchronization region, such as the number of symbols, based on eNBcapability, synchronization signal type, mapped beam dwell time andsweep time, cell-specific distribution of user density, cell-specificdistribution of traffic type, inter-beam interference and other realtime cell-specific statistics.

The eNB may apply synchronization signal type mapping to physicalresources within the eNB transmit beam according to a static andpre-defined configuration, a semi-static configuration by higher layersignaling, or dynamic configuration by higher layer signaling. The eNBmay send semi-static or dynamic synchronization signal type mappingreconfiguration information in a system information broadcast and/ordedicated signaling.

The eNB may transmit a synchronization signal at each symbol location ina synchronization region using a corresponding eNB transmit beam. Themapping of the synchronization signals in the time domain (e.g., tosymbol locations in the synchronization region) and in the spatialdomain (e.g., to the eNB transmit beams) may be as shown in Table 1below. The synchronization signal types may be the same as, or differentbetween, different symbols or different transmit beams.

TABLE 1 Symbol 1 Symbol 2 Symbol 3 Symbol 4 Symbol 5 eNB Synchro-transmit nization beam 1 signal type A eNB Synchro- transmit nizationbeam 2 signal type A eNB Synchro- transmit nization beam 3 signal type AeNB Synchro- transmit nization beam 4 signal type A eNB Synchro-transmit nization beam 5 signal type A

For example, one type of synchronization signal mapping may enable asweeping operation of one synchronization signal type by mapping thesame synchronization signal type to each symbol of the synchronizationregion and to a number of adjacent eNB transmit beams with a combinedcoverage of an entire cell. In an embodiment, such as provided in Table1 above, the eNB may map the same synchronization signal type (such asPSS) to each of the 5 transmit beams and map them to a synchronizationregion in one TTI using consecutive symbols (e.g., each beam with onesymbol dwell time from the beginning of the TTI). This embodiment willbe described in more detail below with respect to FIG. 7 .

In another embodiment, the synchronization signal mapping may be asprovided in Table 2 below. In this embodiment, the eNB may map twosynchronization signal types (e.g., PSS and SSS) to beam 1 and beam 2and allocate them to the synchronization region within one TTI with onebeam having one symbol dwell time and the other having three symboldwell time. This embodiment will be described in more detail below withrespect to FIG. 12 .

TABLE 2 Symbol 1 Symbol 2 Symbol 3 Symbol 4 eNB Synchroni- transmitzation beam 1 signal type A eNB Synchroni- Synchroni- Synchroni-transmit zation zation zation beam 2 signal type B signal type B signaltype B

In embodiments, an eNB may map each synchronization signal according toa specific frequency resource allocation, such as a group ofpre-determined sub-carriers. Synchronization signal types may havedifferent frequency resource allocations based on, for example,scheduling parameters and inter-beam interference. The synchronizationsignal types' frequency and resource allocations may be predefinedand/or signaled in system broadcast information and/or dedicatedsignaling.

The synchronization region may be configured and re-configured based,for example, on changing cell characteristics. In embodiments, thesynchronization region may include a plurality of adjacent symbollocations in a single TTI such that an eNB may transmit the samesynchronization signal type at each consecutive symbol location in a setof adjacent eNB transmit beams with the same dwell time to provideuniform cell-wide coverage.

FIG. 7 is a diagram 700 of example sub-frames that use uniform and fullsweep in one synchronization region. In FIG. 7 , each sub-frame 720A,720B includes a number of TTIs 715A, 715B; 716A, 716B; 717A, 717B. Thesynchronization region 705A, 705B is transmitted periodically every Msubframes and includes five consecutive symbols. In each of theillustrated examples 710 and 750, each of the five symbols is mapped toone of five eNB transmit beams, and the duration of synchronizationregion 705A, 705B is the sum of the dwell time of each used transmitbeam. In the example 710 illustrated in FIG. 7 , the synchronizationregion 705A begins at the beginning of the first TTI 715A in thesubframe 720A. However, as shown in example 750, an eNB may transmit thesynchronization region 705B growing backwards from the end 730 of a TTI(e.g., TTI 715B).

In embodiments, the time resource unit may be a pre-defined time unit,TTI, subframe or frame. The eNB may flexibly adjust the duration of thesynchronization region spanning over the end of the time resource unit,and a WTRU may detect the TTI, subframe or frame start timing based onthe synchronization region and the pre-defined number of base timeresource units (e.g., symbols) for each TTI, subframe or frame. Inembodiments, a WTRU may derive the TTI timing based on the detectedsynchronization signal type and the pre-defined and/or beam-specificsignaled synchronization signal type mapping. For example, a WTRU maydetect, for example using blind detection, a PSS at symbol 3 in TTI 715and may infer the timing of the beginning of the TTI based on thedetected PSS symbol timing and an offset of two-symbol duration. Inother embodiments, a beam-specific symbol distance between the PSS andphysical broadcast channel (PBCH) may be pre-defined, and the WTRU mayfirst decode the PBCH and read the content to determine the beginning ofthe TTI and the TTI number.

In other embodiments, the eNB may use different eNB transmit beams forthe symbols in the synchronization region of each TTI to perform a sweepacross multiple TTIs. FIG. 8 is a diagram 800 of example sub-frames thatuse uniform and full sweep in a multiple-TTI synchronization region805A, 805B. In the illustrated example 802, a two-TTI synchronizationregion 805A is configured. In TTI 815, the eNB transmit beams 1 and 2are mapped to the first two symbols of synchronization region 805A. InTTI 816, the eNB configures a three-symbol region with each symbolmapped to eNB transmit beam 3, 4 and 5. In the illustrated example 804,the synchronization region 805B includes 1 symbol in each of fiveconsecutive TTIs 831, 832, 833, 834, 835. Compared to thesynchronization regions configured in the examples of FIG. 7 , the eNBmay configure a short synchronization region in each TTI, for example,to allow for more symbols for data transmission. However, the resultingsweep to cover the entire cell may take more time and, thus, mayincrease the synchronization latency.

An eNB may dynamically configure the synchronization region with anumber of minimum time resource units (or symbols) based on eNBcapability in terms of the number of supported eNB transmit beams orsystem constraints and requirements regarding how many symbols may beavailable for synchronization regions, the coverage intended by thesynchronization signal type and the distribution of user density andtraffic type intended by the synchronization signal type.

The eNB may reconfigure the synchronization region and dynamicallychange the synchronization signal type mapping to the physical resource.The flexible mapping may provide the eNB with tailored coverage withreduced interference, adaptability to distribution of user density andtraffic type, reduced signaling overhead and lower energy consumption.An eNB may reconfigure the synchronization signal type mapping by, forexample, reducing or increasing the synchronization region with fewer ormore symbols, sweeping eNB transmit beams within one TTI or acrossmultiple TTIs and changing sweep time specific to the mapped eNBtransmit beam, decreasing or increasing the dwell time of each mappedeNB transmit beam by allocating fewer or more symbols in thesynchronization region to the beams (including switching on and off oneor multiple eNB transmit beams), mapping with a new order or pattern ofeNB transmit beams to symbols of the synchronization region, and/orapplying an eNB transmit beam with one or multiple different beamformingproperties (such as beam width, beam gain, beam transmit power, beamshape, and/or beam sidelobe suppression ratio).

FIG. 9 is a diagram 900 of an example synchronization regionre-configuration. In the example illustrated in FIG. 9 , an eNBinitially configures the synchronization region 915A in 910 over 4symbols with respective mapping to transmit beams 1, 2, 3 and 4. In 950,the eNB re-configures the synchronization region 915B by switching thesynchronization in eNB transmit beams 3 and 4 off to reduce thesynchronization region 915 from four symbols to two symbols. This may bedone, for example, due inactivity of users in the beam coverage.

FIG. 10 is a diagram 1000 of another example synchronization regionre-configuration. In the example illustrated in FIG. 10 , the eNBinitially configures the synchronization region 1015A in 1010 with asynchronization mapping only to beam 4 with an increased dwell time of 2symbols, for example, due to reduced activities in the beam coverage. In1050, the eNB re-configures the synchronization region 1015B byswitching on eNB transmit beam 3, for example, due to increased userdensity in eNB transmit beam 3 coverage. In this example, the eNB maykeep the synchronization region duration and reconfigure the dwell timeof eNB transmit beam 4 to allow the addition of eNB transmit beam 3 inthe synchronization region. In embodiments, the eNB may not change thesweep time of the synchronization signal type carried on eNB transmitbeam 4.

As a result of reconfiguring the synchronization region, symbolspreviously used by the removed synchronization signal types may berepurposed. The eNB may maintain the synchronization region length byrepeating one or more synchronization signal types in the same mappedeNB transmit beams in repurposed symbols (e.g., increasing the dwelltime of the mapped synchronization signal types). In other embodiments,the eNB may use these symbols for downlink data channel transmission,for example, the physical downlink shared channel (PDSCH). The eNB mayalso apply discontinuous transmission (DTX) at these symbols to reduceenergy consumption.

The eNB may signal the synchronization signal type mappingreconfiguration in a system information broadcast or using dedicatedsignaling. The WTRU may adjust its synchronization signal type detectionand measurement according to the reconfiguration and may include anyrepurposed symbols for data channel decoding.

An eNB may reconfigure the synchronization signal type mapping by bothchanging the duration of the synchronization region and/or splitting thesynchronization region among a plurality of TTIs. FIG. 11 is a diagram1100 of an example of a flexible mapping and dwell time reconfiguration.In the example illustrated in FIG. 11 , the eNB initially configures asynchronization region 1115A of four symbols in TTI 1120A. In 1150, theeNB splits the synchronization region 1115B into three regions,including one region of one symbol in TTI 1120B mapped to eNB transmitbeam 1, one region of one symbol in TTI 1121 mapped to eNB transmit beam1, and one region of two symbols in TTI 1122 mapped to eNB transmit beam2. The reconfiguration may be done by the scheduler due to variation inthe availability and usage of the eNB transmit beam.

As described above, the eNB may employ a heterogeneous set of transmitbeams having different properties (e.g., beamwidth, transmit power, sidelobe suppression, and/or beam shape) and may map a synchronizationsignal to one or multiple specific transmit beams in the set based onone or more of the properties of the transmit beam. In embodiments,omnidirectional beams may be used, for example, for cell center users orthose WTRUs with high quality radio links. These users may takeadvantage of the omnidirectional synchronization signals, which may betransmitted more frequently, and cell access latency may be reduced.

In embodiments, omni-directional and beamformed synchronization signalsmay reside in different beams but may be placed in the same TTI atdifferent symbol locations or in different TTIs at the same symbollocations. Cell selection criteria for mmW cells may be enhanced byenabling scaling of measured cell receive level based on whether thecorresponding beam type is omni-, wide or narrow, thus ensuringselection of the most appropriate cell for camping.

In connection with beamformed cell access, the selected beam/cell mayhave a direct impact on the random access (RACH) procedure. In mmWsystems, in order for the network to operate efficiently, the mmW cellshould be aware of the appropriate receive beam or beams to use forreceiving RACH transmissions on their corresponding RACH resources. ThemmW cell can assign one or more RACH resource sets to each downlinktransmit beam based on a linkage between the downlink transmit beam andits associated uplink receive beams with corresponding spatial coverage.With flexible cell access, RACH resource configuration, such as preamblesequences, frequency allocation, transmission opportunities, etc., maybe optimized for different beam types based on their specific needs,thus ensuring a high success rate and low latency for different groupsof users. Apart from initial access procedures, neighbor cellmeasurement overhead for mmW systems may be significantly high inconnected mode. The flexibility for mmW cells to transmit multiple beamtypes using different sequence, periodicity, and time-domain placementmay significantly reduce WTRU measurement overhead and improvethroughput without sacrificing required robustness.

FIG. 12 is a diagram 1200 of an example mapping of synchronizationsignals to heterogeneous eNB transmit beams. In the example illustratedin FIG. 12 , the eNB 1205 employs beam 1, which has cell-wide coverage,and beam two, which has part-of-cell coverage. Beam 1 is mapped to thefirst symbol in the synchronization region 1215 and beam 2 is mapped tothe following three symbols in the synchronization region 1215. In anembodiment, the eNB 1205 may map the PSS to the eNB transmit beam 1 withcoverage of the entire cell and the SSS to the narrower transmit beam 2.

The eNB 1205 may also map the symbol locations to the synchronizationsignals according to the transmit beam properties. In the exampleillustrated in FIG. 12 , for example, the PSS may be mapped to onesymbol (with short dwell time) and the SSS may be allocated with threesymbols (with long dwell time). This may be done because, with cell-widecoverage provided by eNB transmit beam 1, a WTRU may receive the PSS ineNB transmit beam 1 more easily compared to SSS carried in eNB transmitbeam 2.

FIG. 13 is a diagram 1300 of an example mapping of synchronizationsignals to heterogeneous eNB transmit beams in different TTIs. In theexample illustrated in FIG. 13 , the synchronization signal types arelocated in a synchronization region 1315 that is split over TTIs 1301and 1303. In embodiments, each synchronization signal type may have adedicated synchronization region in a designed TTI.

In embodiments, such as the example illustrated in FIG. 13 , an eNB mayembed a linkage between different synchronization signal types, and aWTRU may detect one synchronization signal (e.g., PSS) first and use thelinkage information to subsequently locate and detect anothersynchronization signal (e.g., SSS). The linkage information may beexplicitly carried in a data packet or implicitly embedded, such asusing the selected sequence, the applied time and frequency resource, orthe used eNB transmit beam. The linkage information may include, forexample, symbol distance to the linked synchronization signal, eNBtransmit beam index of the transmit beam used by the synchronizationsignal, frequency resource offset to the linked synchronization signaland/or sequence index used for the linked synchronization signal.

In another example, multiple synchronization signal types may betransmitted at the same time resource unit (e.g., same symbol). Onesynchronization signal type may use a wide eNB transmit beam that mayprovide cell-wide coverage, and another co-located synchronizationsignal type may use a narrow eNB transmit beam covering a part of thecell. The co-located synchronization signal types may employ differentsequences and/or frequency resources for WTRUs to detect.

FIG. 14 is a diagram 1400 of an example mapping of synchronizationsignal types to heterogeneous eNB transmit beams at the same symbols. Inthe example illustrated in FIG. 14 , an eNB 1405 may map the PSS and/orSSS in both the transmit beam 1410 and the transmit beam 1415 in thesame first four symbols of TTI 1401. Transmit beam 1410 may providecell-wide coverage and transmit beam 1415 may have improved link budgetwith higher beamforming gain and with sweeping coverage.

A WTRU may detect both synchronization signal types at the same symbollocations using a different sequence for detection or at differentfrequency resources. The WTRU may select between the detectedsynchronization signal types according to pre-defined and/orpre-configured rules. For example, the cell-center WTRUs may detectmultiple synchronization signal types due to low path loss to the eNB,and the WTRUs may select the synchronization signal type carried in theeNB transmit beam covering the entire cell so that the followingbroadcast channel decoding may not require beam sweeping and pairingand, thus, may have lower latency. In this case, the selectedsynchronization signal type may not have the highest energy detected atthe WTRUs but may have energy above a pre-defined threshold. Cell-edgeWTRUs may detect multiple synchronization types, and they may select thesynchronization signal types with the highest detected energy in orderto more successfully detect the following broadcast channel associatedwith the synchronization signal type.

A broadcast channel may provide all necessary information specific to acell and/or the beam that may carry the broadcast channel in order for aWTRU to gain access to the cell. The information content of thebroadcast channel may be referred to as master information block (MIB)information. A beamformed broadcast channel may be transmitted usingdigital and/or analog beamforming to provide improved broadcast channellink performance. The formed eNB transmit beam may cover an entire cell,or part of the cell, depending on the beamforming weights applied to thebroadcast channel. In the embodiments described herein, the broadcastchannel may be, for example, a PBCH.

Cell-specific and/or beam-specific information carried in the beamformedbroadcast channel may include, for example, beam information, cellinformation, timing information, and/or associated control channellinkage. Beam information may include, for example, beam identity,number of beams of the cell, beam dwell time, beam sweep time, beamsequence index, beam sweep/schedule, and/or beam scrambling. Cellinformation may include, for example, cell system bandwidth and/orsystem frame number (SFN). Timing information may include allinformation necessary for WTRUs to determine various timing of the cell,beam and associated channels, for example. Such timing information mayinclude, for example, TTI number, subframe number, frame number, systemnumber, timing offset in terms of time resource units (e.g., number ofsymbols between the broadcast channel and the start of theTTI/subframe/frame or any combination thereof), and/or timing offset interms of number of time resource units (e.g., number of symbols betweenthe broadcast channel and its associated control channels, includingdownlink link format, downlink control channel, and/or downlink HARQfeedback channel).

Associated control channel linkage may include, for example, allinformation necessary for WTRUs to locate and demodulate the associatedcontrol channels and data or shared channels. A resource allocation andconfiguration of the control channel associated with the broadcastchannel, such as the PDCCH, may be co-located in the same eNB transmitbeam and may include, for example, the size of the control channelregion in terms of number of symbols and a frequency resourceallocation. The associated control channel may use another eNB transmitbeam, and the broadcast channel may provide the beam information listedabove for the control channel beam. When the control channel applies adifferent reference signal for broadcast channel demodulation, theassociated control channel linkage may include a resource allocation andconfiguration of the reference signal used to demodulate the associatedcontrol channel. The information may include, for example, the referencesignal type, sequence length, symbol location, and/or frequency resourceallocation. In embodiments, the broadcast channel may containinformation for mapping the beam-specific reference signal to theassociated control channel within the beam. The configuration of thecontrol channel may also include fixed and flexible mapping to thephysical resource, such as the number of symbols used for the controlchannel and/or the index of the beam carrying the control channel.

The associated channel linkage may also include resource allocation andconfiguration of the format indication channel, such as the physicalcontrol format indicator channel (PCFICH) where the associated controlchannel configuration may be found. The configuration may includebeam-specific information, such as beam index and beam specificreference signal for the format indication channel demodulation. Theassociated channel linkage may also include resource allocation andconfiguration of a downlink HARQ feedback channel, such as the PHICHwhere downlink acknowledgement/negative acknowledgement (ACK/NACK) maybe transmitted. The configuration may include beam information specificto the downlink HARQ feedback channel, such as the beam index andbeam-specific reference signal for HARQ feedback channel demodulation.

In a beamformed system, an eNB may transmit multiple beamformedbroadcast channels for one cell and transmit a different part of MIBinformation in each beamformed broadcast channel. FIG. 15 is a diagram1500 of an example mapping where the MIB is divided such that the beaminformation (also referred to as pre-MIB) is transmitted on a cell-wideeNB transmit beam and the rest of the MIB is transmitted in a narrowtransmit beam. In the example illustrated in FIG. 15 , an eNB 1501 isconfigured with a wide beam 1502 and a narrow beam 1503. Using a mappingsuch as illustrated in FIG. 15 , a WTRU may first receive one or moresynchronization signal types (e.g., PSS/SSS) in synchronization region1505. The synchronization region 1505 may be mapped to any of the wideor narrow beams, as described above. A WTRU may use linkage informationobtained from a synchronization signal, or pre-configured information,such as a symbol distance offset 1525 and/or a symbol distance offset toTTI start 1520, to detect and decode a first (wide-beam) PBCH region1510. The linkage between the synchronization region 1505 and the firstPBCH region 1510 is illustrated in FIG. 15 by the arrow 1535.

The WTRU may receive pre-MIB in the cell-wide broadcast channel and usethe pre-MIB information to detect and decode the next beamformed channelin a second PBCH region 1515 mapped to the narrow eNB transmit beam1503. For example, the WTRU may use a symbol offset between the firstPBCH region 1510 and the second PBCH region 1515 to detect and decodethe second PBCH region 1515. The linkage between the two PBCH regions isillustrated by the arrow 1540 in FIG. 15 . This hierarchical broadcastchannel configuration may significantly reduce the time WTRUs may use tosweep and pair beams to receive the beamformed broadcast channel in thenarrow eNB transmit beam.

A beamformed broadcast channel may be associated with differentsynchronization signal types and, thus, different eNB transmit beams.There may be an explicit or implicit linkage between the synchronizationsignal and the associated broadcast channel beam. A beamformed broadcastchannel may be associated with a synchronization signal type in aone-to-one mapping or in a one-to-many mapping.

FIG. 16 is a diagram 1600 of an example mapping where the broadcastchannel is associated with a synchronization signal type in a one-to-onemapping. In the example illustrated in FIG. 16 , the synchronizationsignal type in the synchronization region 1605 and associated broadcastchannel in the broadcast region 1610 are carried in the same eNBtransmit beam. However, in embodiments, the synchronization types may beassociated with broadcast beams in different transmit beams.

There may be a fixed beam-specific offset in terms of number of timeresource units within the same beam between the broadcast channel andthe associated synchronization signal. The offsets may be identical foreach synchronization signal type and broadcast channel pair located inthe same beam. The fixed offset may be different between synchronizationsignal type and broadcast channel when they are carried in different eNBtransmit beams.

In embodiments, the beamformed broadcast channel may have a variablesymbol location within a TTI, subframe or frame. In these scenarios,WTRUs may apply blind decoding to locate and decode the broadcastchannel.

FIG. 17 is a diagram 1700 of an example mapping where the broadcastchannel is associated with a synchronization signal type in aone-to-many mapping. In the example illustrated in FIG. 17 , thebroadcast channel may be carried in a cell-wide beam 1710 in broadcastregion 1704, 1706 and may be linked to each synchronization signal typecarried in a different eNB narrow transmit beam such as beam 1708 insynchronization region 1702. The symbol distance offset between thebroadcast channel and each synchronization signal type may be different,and WTRUs may apply blind decoding to locate and decode the broadcastchannel.

In embodiments, a WTRU may use a pre-defined set of values to determinea cell-wide beam broadcast channel location based on the detectedsynchronization signal location. The offset may indicate, for example,the symbol distance between the detected synchronization signal type andassociated broadcast channel in the same wide or narrow eNB transmitbeam or the symbol distance between the detected synchronization signaltype and associated broadcast channel in a different wide or narrow eNBtransmit beam. The offset may take into account that the periodicity andsymbol locations of the broadcast channel in the cell-wide and narrowtransmit beams may be different.

In embodiments, the offset values may be pre-defined or pre-configuredor may be obtained by blind decoding. Additionally, the offset valuesmay be indicated by one or a few properties of the associatedsynchronization signal types. The synchronization signal type,therefore, may have one or multiple properties that may indicate linkageinformation between the synchronization signal type and associatedbroadcast channel in order to locate and decode the broadcast channel.The properties may include the synchronization signal type sequenceindex, time resource allocation (such as symbol location, TTI number,subframe number and/or frame number), frequency resource allocation(such as radio bearer (RB) number) and spatial resource allocation (suchas beam index). The linkage information may be based on this informationand a pre-defined or pre-configured mapping or table.

The linkage between the synchronization signal type and associatedbeamformed broadcast channel may also include link adaptationinformation of the broadcast channel. For example, the linkageinformation may indicate the transport format of the broadcast channelsuch as the coding and modulation used and also the periodicity of thetransmission. For example, a broadcast channel carried in a wide eNBtransmit beam may apply a conservative transport format and a lowtransmission interval to ensure reliability of PBCH decoding. Abroadcast channel associated with synchronization signal types in anarrow eNB transmit beam may use an aggressive transport format to carrymore system information.

A beamformed broadcast channel may be multiplexed with a beam-specificand/or cell-specific reference signal used by WTRUs to de-modulate thebroadcast channel. A reference signal with the same beamformingconfiguration as the broadcast channel may be located at a fixed offsetin terms of number of time resource units (e.g., number of symbols fromthe linked synchronization signals).

The reference signal may be used to demodulate the beamformed broadcastchannel. It may be a cell-specific reference signal and may be used byall beams carrying broadcast channels. The reference signal may bescrambled with a beam identity and may be transmitted using abeam-specific frequency or code resource determined by the beam index oridentity.

A synchronization signal may provide information to determine thereference signal configuration for a WTRU to apply the reference signalto demodulate the broadcast channel data. The broadcast channel may usea pre-defined sequence mapping to select the used sequence based on oneor a few properties of the associated synchronization signal types.

For example, there may be a table between the synchronization signaltype sequence index and the associated broadcast channel referencesignal sequence index. In another example, the broadcast channelreference signal sequence may be a function of the sequence index and/orthe time or frequency resource allocation, such as the symbol locationor RB number of the detected synchronization signal type. WTRUs may usethe function to determine the broadcast channel reference signalconfiguration.

The PBCH may provide mapping information for BRS to the linked physicaldownlink control channel (PDCCH) beam. The same reference signal may beused for the PFICH and the PDCCH, and the PBCH may have information toindicate the PCFICH resource allocation size and PDCCH configuration perbeam.

The WTRU may use different methods of initial cell search to detect thesynchronization signals and decode the broadcast channel. Inembodiments, hierarchical cell search may be used.

FIG. 18 is a flow diagram 1800 of an example method of initial cellsearch and selection using beamforming. In the example illustrated inFIG. 18 , a WTRU detects a synchronization signal (1810). In anembodiment, this may be done by sweeping a respective one of a pluralityof receive beams during each of a plurality of synchronizationsub-frames, using a pre-defined sweep time and dwell period, to detect asynchronization signal. A WTRU may, for example, distinguish asynchronization signal type and its associated broadcast channel using aproperty of the detected signal. In the example illustrated in FIG. 18 ,the WTRU obtains symbol timing information and a synchronization signalindex from the detected synchronization signal (1820). The WTRU may beconfigured with a set of synchronization signal indices, and theobtained synchronization signal index may correspond to asynchronization signal index in the set. The WTRU may decode a firstbroadcast channel using the obtained symbol timing information, theobtained synchronization signal index and a predefined or blind-decodedsymbol distance between the detected synchronization signal and thefirst broadcast channel (1830). The WTRU may decode a second broadcastchannel using information obtained from decoding the first broadcastchannel (1840).

In embodiments, a specific set or subset of sequences may be used onlyfor synchronization signals used in a wide beam to provide for widecoverage, and different sets of the sequences may be used for thesynchronization signals used in narrow beams. In embodiments, the WTRUmay detect different synchronization signal types based on the frequencyresource or time resource allocation of the detected synchronizationsignal.

To detect the synchronization signal, a WTRU may apply a hierarchicalcell search to sequentially search a set of synchronization signals withdecreasing associated bandwidth. Each synchronization signal type mayuse a specific sequence and link with the next synchronization signaltype. For example, a WTRU may identify the PSS based on its sequence IDand may use link information provided in the PSS to detect the SSS. AWTRU may apply a pre-defined accumulation scheme for eachsynchronization type, for example.

In embodiments, the WTRU may obtain a metric from the detectedsynchronization signal for each of the plurality of receive beams. TheWTRU may identify one of the plurality of receive beams that has thebest metric and obtain a sweep time and dwell period for the identifiedone of the plurality of receive beams. The WTRU may identify a set ofreceive beams within the identified one of the plurality of receivebeams and sweep a respective one of the set of receive beams during eachof a plurality of synchronization sub-frames using the obtained sweepand dwell period. The WTRU may detect a synchronization signal using thesweeping. This may be used with a staged mmW search procedure, such asis described below with respect to FIG. 19 .

In embodiments, a WTRU may select a synchronization signal type that isabove a pre-defined threshold and use the linkage information to decodethe associated broadcast channel in the linked eNB transmit beam toacquire the cell access. A WTRU may use the beam scheduling and sweepinformation acquired in the selected eNB transmit beam and repeat thesynchronization of other synchronization signal types in different eNBtransmit beams to evaluate all available eNB transmit beams in the celland may select another one for further cell access. The selectioncriteria may be a measured beam-specific reference signal qualitymetric.

In embodiments, WTRUs may receive a synchronization signal typereconfiguration signaled in a system information broadcast or dedicatedsignaling intended for beam selection/reselection and measurement. Theconfiguration may include information associated with a plurality ofdownlink directional beams, and the information may include at leastsynchronization signal types associated with each of the plurality ofdownlink directional beams and a configuration of each synchronizationsignal type. The configuration may include, for example, the size of theunique word used in the associated eNB directional beam, the type of theunique word used in the associated eNB directional beam, and the indexand identity of the unique word used in the associated eNB directionalbeam.

A WTRU may sweep the receive beam and detect the synchronization signaltypes, each of which may be associated with an eNB downlink directionalbeam, as mentioned above. The WTRU may select a detected eNB downlinkdirectional beam for synchronization and reception according to apre-configured criteria, such as WTRU service type, WTRU capability,and/or synchronization signal type. The WTRU may synchronize with theselected synchronization signal type within its associated eNB downlinkdirectional beam and receive communication within the selected eNBdownlink directional beam.

In embodiments, a WTRU may synchronize with a cell using onesynchronization signal type and decode the cell broadcast channel andestablish symbol, TTI, sub-frame and SFN timing. In embodiments, theWTRU may detect the presence of one synchronization signal type (e.g.,PSS) in a time window by correlating one or multiple pre-definedsequences specific for the sought synchronization type. The sequencesmay have pre-defined properties that may indicate one or more of thesymbol distance between the synchronization signal type and anothersynchronization signal type, the symbol distance between thesynchronization signal and the broadcast channel, TTI number and cellidentity and/or beam identity information, the broadcast channeldemodulation reference signal sequence, presence of the broadcastchannel mapped to the same synchronization signal type eNB transmit beamand associated broadcast channel transport format.

The WTRU may acquire the symbol timing, for example, at the highest peakper sequence (e.g., synchronization signal type), which may exceed apre-defined threshold. The WTRU may select a synchronization signal typeaccording to pre-defined and/or pre-configured rules, such as apreference for synchronization signal types carried in a cell-wide eNBtransmit beam to reduce the latency of the broadcast channel decoding,preference for synchronization signal types carried in a highlybeamformed transmit beam to improve the decoding performance of thebroadcast channel coding, and preference for synchronization signaltypes based on the WTRU service type, traffic type and WTRU capability.

The WTRU may establish reference symbol timing start based on thedetected and selected synchronization signal type. The WTRU may decodethe broadcast channel using the linkage information derived from thedetected synchronization signal type, such as the symbol distanceoffset, broadcast channel reference sequence index and broadcast channeltransport format. The WTRU may acquire broadcast channel content of MIBor pre-MIB, including all cell, beam, timing, control channel and otherinformation, such as offset to TTI start (e.g., symbol number, SFN, oreNB transmit beam identity), number of supported eNB transmit beams, andother beam-specific and/or cell-specific configuration. When receivingpre-MIB, the WTRU may use the linkage information to decode the nextbroadcast channel and acquire the cell system information.

In embodiments, the WTRU may locate the control channel beam using thebroadcast information and may acquire the control channel. A WTRU maysweep receive beams and search one or multiple synchronization signaltypes using their predefined configuration. The sought synchronizationsignal types may be determined by service type, WTRU category andcapability. The WTRU may acquire time resource unit timing, such assymbol timing, from one synchronization signal type and linkageinformation to another synchronization signal type. The WTRU may detectanother synchronization signal to acquire TTI timing, sub-frame timingand/or frame timing and use the linkage between the synchronizationsignal and its associated broadcast channel to locate the broadcastchannel to read beam-specific information, including, for example, beamidentity, number of beams, sweep schedule and other system information.The WTRU may then locate a control channel beam using the broadcastinformation and may acquire the control channel and read systeminformation.

Regarding WTRU rotation, to better illustrate the importance ofconsidering rotational motion over translational motion, a briefderivation of their relationship in the context of aligning WTRU and eNBbeams is described. For sake of simplicity, a 2D example is considered,where the eNB and WTRU are beam aligned on the x-axis and separated by adistance d. Assuming the WTRU moves in the positive y direction with aspeed of v(t) km/h, the following may be true:

$\begin{matrix}{{\tan( {\theta(t)} )} = \frac{y(t)}{d}} & (1)\end{matrix}$

In equation (1), θ may be the LOS angle between the eNB and the WTRU,and y(t) may be the vertical distance at time t. Using the method ofimplicit differentiation on (1) gives the following:

$\begin{matrix}{{( \frac{d{\theta(t)}}{dt} )se{c^{2}( {\theta(t)} )}} = {\frac{1}{d}( \frac{d{y(t)}}{dt} )}} & (2)\end{matrix}$which may be arranged to obtain the linear speed,

${{v(t)} = \frac{{dy}(t)}{dt}},$as a function of the angular speed,

${\omega(t)} = {\frac{d{\theta(t)}}{dt}.}$v(t)=dω(t)sec²(θ(t))  (3)

From (3), it can be seen that, in order to maintain a constant angularspeed, the linear speed must continually increase. Furthermore, the rateof the increase also increases both with the angle, θ and the distance,d. It was discovered via experimentation that rotation speeds fororientation changes (e.g., flipping a phone up from a table for viewing)are in the range of 45° to 360° per second. This range is in line withother independent investigations, which go further to provide estimatesof rotational speeds up to 800° per second for gaming uses. Assumingd=25 m, and using a modest angular speed of 45° per second, theequivalent linear speed may be approximately 70 and 140 km/h forinstantaneous angles of 0° to 45°, respectively. This simple examplemakes clear that rotational motion in mmW communication may have a muchgreater impact than translational motion for maintaining beam alignment.

Two main algorithms were studied for purposes of maximizing a functionthat represents the link quality as a function of the beam pair for eachof K cells: an exhaustive search and a staged search. FIG. 19 is adiagram 1900 showing examples of an exhaustive search and a stagedsearch.

An example exhaustive search procedure is illustrated at 1910 in FIG. 19. In the example 1910 illustrated in FIG. 19 , a WTRU may search overall eNB transmit beams 1904-1908 from all cells by sweeping one of Nreceive beams 1912 during each synchronization subframe 1902. The totalsearch time will, therefore, be N frames (assuming one synchronizationsubframe per downlink frame). At the end of the search, the followingmaximization may be evaluated:

$\begin{matrix}{\hat{k},\hat{m},{\hat{n} = {\underset{k,m,n}{argmax}\{ {f( {k,m,n} )} \}}}} & (4)\end{matrix}$

In equation (4), k, m, and n are the cell index, cell beam index, andWTRU beam index, respectively, {circumflex over (k)}, {circumflex over(m)}, and {circumflex over (n)} are the corresponding chosen indices,and f(.) is the objective function representing the quality of thereceived synchronization signal. For purposes simulation, the objectivefunction will be a signal to interference plus noise ratio (SINR)measurement.

An example staged procedure is illustrated at 1920 in FIG. 19 . In theexample illustrated in FIG. 19 , a WTRU is defined to have N_(w) widebeams 1928 and may have the capability to generate Nt thinner beams 1928within each of the N_(w) wide beams 1912. The total number of WTRUreceive beams is still N, which may be defined as N=N_(w)N_(t).

A staged procedure may begin with a WTRU searching over all M transmitbeams 1924-1926 by sweeping one of the N_(w) receive beams 1928 duringeach synchronization subframe 1922. At the end of the first searchstage, the following maximization may be evaluated:

$\begin{matrix}{\hat{k},\hat{m},{= {\underset{k,m,n_{w}}{argmax}\{ {f( {k,m,n_{w}} )} \}}}} & (5)\end{matrix}$

In equation (5), k, m and n_(w) are the cell index, cell beam index andWTRU wide beam index, respectively, and {circumflex over (k)},{circumflex over (m)}, and {circumflex over (n)} are the correspondingchosen cell beam indices. Although {circumflex over (m)} is the chosencell beam index, the second stage will repeat the search over all Mbeams for the chosen cell.

In the second stage, since the WTRU has already identified the cellindex, {circumflex over (k)}, and a WTRU wide beam index,

, a reduced search may take place using only

$N_{t} = \frac{N}{N_{w}}$receive beams 1928 to search over the M transmit beams 1924-1926 duringeach synchronization subframe 1922. At the end of the second stage,another maximization may be evaluated:

$\begin{matrix}{\hat{m},{= {\underset{m,n_{t}}{argmax}\{ {f( {m,n_{t}} )} \}}}} & (6)\end{matrix}$

In equation (6), n_(t) is the second stage WTRU thin beam index, m isthe cell beam index,

is the chosen WTRU array beam index and {circumflex over (m)} is thechosen cell beam index. Using the same synchronization subframeperiodicity as in the exhaustive procedure, the total search time forthis approach is then

$( {N_{w} + \frac{N}{N_{w}}} )$frames.

Both the exhaustive and staged search procedures may assume that oneobservation per beam pair is made before the maximization. Timediversity, in the form of averaging multiple observations over time, issometimes used in LTE-based cell search to combat fast fading. However,because of the presumed higher sensitivity to WTRU rotational motion,the studies described herein use single observation methods only.

The impact of WTRU rotational motion using both the exhaustive andstaged searching procedures was analyzed using a custom Matlab basedsystem simulation environment wherein WTRUs experience bothtranslational and rotational motion based on custom models. Overallparameters for the simulations are provided in Table 3 below.

TABLE 3 Parameter Values(s)/Description AWE WinProp Ray FracingParameters Environment METIS Madrid Building Material- RelativePermittivity: 6 Concrete Relative Permeability: 1 Conductivity: 1 S/mVehicle Material- Relative Permittivity: 1 Metallic RelativePermeability: 20 Conductivity: 1000 S/m Cars: 5.0 × 2 × 1.5 mrectangular prism Trucks: 14 × 2 × 3.7 m rectangular prism SUVs: 5.3 ×2.1 × 2 m rectangular prism Human Blockers Relative Permittivity: 24Relative Permeability: 1 Conductivity: 10 S/m Vegetation/TreesPenetration Loss: 3.21 dB/m Grid Resolution 3 m mB/UE Height 5/1.5 mFrequency F_(c) = 28 GHz mB Tx Power 40 dBm EIRP mB Density 45 mB/km²Antenna Configuration Parameters mB 3 dB Beam Widths 15°, 6 beams perarray WTRU 3 dB Beam Widths [15°, 8°], [6, 12] beams per array SystemSimulation Parameters Bandwidth B = 500 MHz Noise Power per Hz −174dBm/Hz WTRU Density 5000 WTRU's/km² Number of Drops 25 Initial Device Inthe horizontal plane with a Orientation uniformly random rotation aroundthe z-axis. WTRU Translation Each WTRU is assigned a speed Motion[km/hr] from a uniform distribution with values between [3-30] as wellas a direction from one of the four cardinal directions. UE RotationValues vary between [0-800], and Motion [deg/sec] are based on the testcase being run.

The simulation results highlight the performance difference between theexhaustive and staged procedures as well as the impacts from WTRUmotion. A first set of simulations were performed for rotating andnon-rotating WTRUs to highlight the impact of rotational motion. Thetest case specific simulation parameters are listed in Table 4, and theresults are shown in the graph 2000 in FIG. 20 . The simulations assumeone wide beam per array.

TABLE 4 WTRU # WTRU Rotation Test Beams per Procedure Procedure TimeSpeed Number Array Type [# of TTIs] [deg/sec] 1 6 Exhaustive 240 0 2 6Staged 100 0 3 12 Exhaustive 480 0 4 12 Staged 160 0 5 6 Exhaustive 240360 6 6 Staged 100 360 7 12 Exhaustive 480 360 8 12 Staged 160 360

Table 5 shows the mean SINR differences between the exhaustive andstaged procedures both with and without WTRU rotation. The first twocolumns indicate that when there is no rotation, the staged approach maybe preferable since it results in less processing time and energyconsumption and further shows negligible performance differences. Thestaged approach is a non-exhaustive approach, which classically comes atthe cost of some amount of performance degradation. In this case,however, the methodology used results in mostly identical beams beingchosen for both procedures. Hence, there is minimal performance impact.The second two columns, which represent cases where the WTRU is underrotational motion, go even further to show that there is even a gainincurred by using the non-exhaustive search procedure. This may beattributed to the fact that the SINRs measured during the procedure whenthe device is being rotated may become stale by the end of the procedurewhen the maximization is evaluated. This further points to a potentialadvantage to using a staged approach for mmW systems.

TABLE 5 # WTRU Beams per Array 6 12 6 12 WTRU Rotation Speed [deg/sec] 00 360 360 Mean SINR Delta [dB] 0.37 0.58 −1.32 −7.2

Table 6 shows the mean SINR differences between non-rotating androtating WTRUs and shows that, for both WTRU beam widths, theperformance is more severely impacted by rotation in the exhaustivesearch compared with the staged search. This may highlight the fact thatimpact from rotation is more directly tied to the required procedureprocessing time relative to the WTRU rotational speed.

TABLE 6 # WTRU Beams per Array 6 12 6 12 Procedure Exhaus- Exhaus-Staged Staged tive tive Mean SINR [dB] 1.9 9.75 0.21 2.0

The first set of simulations highlights the need to consider WTRUrotation when designing system procedures for a mmW system. A second setof simulations was performed to explicitly show the performance as afunction of the rotational speed. The exhaustive and staged procedureswere simulated, both using WTRUs with 12 beams per array. Each of thesetwo configurations was tested for rotation speeds from 0° to 800° persecond. The mean SINR was subtracted from each test and plotted as shownin the graph 2100 in FIG. 21 . The SINR loss per degree/sec of rotationwas extracted by fitting a line to the two curves and was found to be 1dB per 100°/sec and ˜2 dB per 100°/sec for the staged and exhaustiveprocedures respectively.

Although features and elements are described above in particularcombinations, one of ordinary skill in the art will appreciate that eachfeature or element can be used alone or in any combination with theother features and elements. In addition, the methods described hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable medium for execution by a computeror processor. Examples of computer-readable media include electronicsignals (transmitted over wired or wireless connections) andcomputer-readable storage media. Examples of computer-readable storagemedia include, but are not limited to, a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs). A processor in association withsoftware may be used to implement a radio frequency transceiver for usein a WTRU, UE, terminal, base station, RNC, or any host computer.

What is claimed is:
 1. A base station comprising: an antenna; atransceiver, operatively coupled to the antenna; and a processor,operatively coupled to the transceiver; the antenna, the transceiver,and the processor configured to transmit a configuration messageincluding at least information indicating a subset of a plurality ofbase station transmit beams to be used, by the base station, fortransmitting a set of synchronization signals; the antenna and thetransceiver configured to transmit the set of synchronization signals,wherein the set of synchronization signals includes a primarysynchronization signal and a secondary synchronization signal; theantenna, the transceiver, and the processor configured to transmit areference signal along with a physical broadcast channel (PBCH)transmission, wherein the reference signal has a sequence that isderived from a beam index associated with one of the subset of theplurality of base station transmit beams used by the base station totransmit a set of synchronization signals; and the antenna and thetransceiver configured to receive a random access channel (RACH)transmission from a wireless transmit/receive unit (WTRU), wherein theRACH transmission includes a preamble sequence associated with the oneof the subset of the plurality of base station transmit beams used bythe base station to transmit the set of synchronization signals.
 2. Thebase station of claim 1, wherein the reference signal is offset from theprimary synchronization signal by a fixed number of symbols.
 3. The basestation of claim 1, wherein the transceiver is configured to transmit anindication in a system information block transmission of resources to beused for transmission of the set of synchronization signals.
 4. The basestation of claim 1, wherein the set of synchronization signalstransmitted by the antenna and the transceiver using the one of thesubset of the plurality of base station transmit beams has a highestsignal quality among sets of synchronization signals transmitted by thebase station using the subset of the plurality of base station transmitbeams.
 5. The base station of claim 1, the antenna, the transceiver, andthe processor configured to transmit information indicating criteria tobe used, by the WTRU, for selecting the set of synchronization signals.6. The base station of claim 5, wherein the criteria to be used by theWTRU, for selecting the set of synchronization signals, is based on asignal quality metric.
 7. The base station of claim 1, wherein the setof synchronization signals is transmitted at least twice using atransmission periodicity.
 8. The base station of claim 7, whereininformation indicating the transmission periodicity is included in theconfiguration message.
 9. The base station of claim 1, wherein theantenna, the processor, and the transceiver are configured to sweep oneof a plurality of base station transmit beams during each of a pluralityof synchronization sub-frames, using a pre-defined sweep time and dwellperiod, to transmit another set of synchronization signals.
 10. The basestation of claim 1, wherein the PBCH transmission provides system timinginformation, a beam-specific reference sequence, and a beam-specificresource allocation associated with another PBCH transmission andwherein the system timing information indicates at least a beam sweeptime and a dwell period of a transmission of the another PBCHtransmission.
 11. A method performed by a base station, the methodcomprising: transmitting a configuration message including at leastinformation indicating a subset of a plurality of base station transmitbeams to be used, by the base station, for transmitting a set ofsynchronization signals; transmitting the set of synchronizationsignals, wherein the set of synchronization signals includes a primarysynchronization signal and a secondary synchronization signal;transmitting a reference signal along with a physical broadcast channel(PBCH) transmission, wherein the reference signal has a sequence that isderived from a beam index associated with one of the subset of theplurality of base station transmit beams used by the base station totransmit a set of synchronization signals; and receiving a random accesschannel (RACH) transmission from a wireless transmit/receive unit(WTRU), wherein the RACH transmission includes a preamble sequenceassociated with the one of the subset of the plurality of base stationtransmit beams used by the base station to transmit the set ofsynchronization signals.
 12. The method of claim 11, wherein thereference signal is offset from the primary synchronization signal by afixed number of symbols.
 13. The method of claim 11 comprisingtransmitting, in a system information block transmission, informationindicating resources to be used for transmission of the set ofsynchronization signals.
 14. The method of claim 11, wherein the set ofsynchronization signals transmitted by the base station using the one ofthe subset of the plurality of base station transmit beams has a highestsignal quality among sets of synchronization signals transmitted by thebase station using the subset of the plurality of base station transmitbeams.
 15. The method of claim 11 comprising transmitting informationindicating criteria to be used, by the WTRU, for selecting the set ofsynchronization signals.
 16. The method of claim 15, wherein thecriteria to be used the WTRU, for selecting the set of synchronizationsignals, is based on a signal quality metric.
 17. The method of claim11, wherein the set of synchronization signals is transmitted at leasttwice using a transmission periodicity.
 18. The method of claim 17,wherein information indicating the transmission periodicity is includedin the configuration message.
 19. The method of claim 11 comprisingsweeping one of a plurality of base station transmit beams during eachof a plurality of synchronization sub-frames, using a pre-defined sweeptime and dwell period, to transmit another set of synchronizationsignals.
 20. The method of claim 11, wherein the PBCH transmissionprovides system timing information, a beam-specific reference sequence,and a beam-specific resource allocation associated with another PBCHtransmission and wherein the system timing information indicates atleast a beam sweep time and a dwell period of a transmission of theanother PBCH transmission.